The present invention relates to vacuum deposition of a thin film onto a substrate and, more particularly, to cathode sputtering.
Sputtering involves an evacuated vessel, or vacuum chamber, containing an anode and a cathode across which a potential drop is applied by a power supply. The cathode is either made of the material to be deposited or is overlaid with a target material to be deposited. The substrate, on which the thin film of material from the cathode is to be deposited, is affixed to the face of the anode or placed generally near the cathode. A relatively unreactive sputtering gas, such as argon, is introduced to the system. The electric field caused by the potential drop between electrodes causes the sputtering gas to break down into positive ions and electrons. The ions impact the cathode, causing atoms of the target material to be liberated therefrom, resulting in the deposition of material on the substrate. Thin films, typically less than 100 micrometers thick, are produced. A glow discharge occurs during deposition as a result of electrons produced at the cathode by positive ion bombardment. The electrons are driven toward the anode, thereby producing more electrons and producing more positive ions to bombard and erode the cathode. The electrons also excite gas atoms, gas molecules, and the sputtered atoms and particles in transit from the target to the substrate. As this excited matter falls back to lower energy states, photons are emitted. These photons are observed as the glow discharge.
A type of sputtering to which the present invention is particularly adaptable is known as reactive sputtering. In reactive sputtering, the inert sputtering gas is mixed with a small quantity of reactive gas. The gas chemically reacts with the surface of the cathode and with the film surface. In this manner, thin films of carbides, nitrides, oxides, hydrides, sulfides, etc. of target materials such as tantalum, aluminum, lead, silicon, Al-Si alloys, ferrites, chromium, beryllium, and germanium can be produced. This process can be employed with either the glow discharge apparatus described above, or with an rf sputtering arrangement as is known in the art. Reactive sputtering is potentially particularly advantageous for use in producing thin films, but a major disadvantage has been the difficulty in controlling the composition of the products. The product films desired in reactive sputtering are frequently those which are "intermediately reacted", that is, they are not reacted to their full extent as would be indicated by their valence.
For example, an intermediately reacted film of silicon oxide can be formed by sputtering silicon in a small pressure of oxygen. A target plate of the element silicon is attached to a cathode assembly, a high vacuum is obtained, a small pressure of argon gas is introduced, and a negative potential of several hundred volts is applied to the cathode. A glow discharge results and a dark, metallic, amorphous, semiconducting film condenses on a substrate placed near the cathode. One desirable property is a certain index of refraction, n, for some optical application, such as in an anti-reflecting film on a silicon photo-voltaic cell. If a small pressure of oxygen is added, a brown insulating film is obtained, and, if a greater oxygen pressure is added, a clear insulating film results. These films are generally described by the notation SiO.sub.(2-X), where SiO.sub.2 is generally accepted as the completely oxidized compound and X between zero and two indicates the lack of oxygen in the suboxidized compound. The following values for an intermediate SiO compound are reported by K. L. Chopra, Thin Film Phenomena, 1969, McGraw Hill, p. 750:
Table I ______________________________________ Compound X n Wave length (microns) ______________________________________ Si 2 3.3 2.0 SiO approx. 1 1.55-2.0 0.55 SiO approx. 1 1.5-1.8 2.0 SiO.sub.2 0 1.44 0.55 SiO.sub.2 0 1.46 2.0 ______________________________________
Clearly, the control of desirable film properties requires close control of the amount of oxygen added to the sputtering vessel. Another example of close control is the formation of titanium carbide, TiC.sub.(1-X), by sputtering titanium metal in the presence of argon and methane (CH.sub.4). (Titanim hydride decomposes above 400.degree. C., so the substrate may be heated to inhibit hydride formation.) Titanium carbide is known to exhibit a range of mechanical properties such as hardness over its range of X (0 to 1), making close control of the extent of reaction again desirable.
A number of drawbacks are inherent in prior art devices for controlling vacuum deposition. E. Kay, in U.S. Pat. No. 3,354,074, suggests the use of a "film thickness profile" to test the uniformity of film growth. This technique relies on preselected control settings. Another prior art device employs a feedback controlled power supply with a thickness and rate control system in which a quartz crystal oscillator is located near the target so that film is deposited on the crystal's front electrode. The mass of film added to the crystal's front electrode changes the crystal's oscillation frequency. This frequency change is measured and correlated to film thickness. By taking the time derivative electronically, the deposition rate is obtained. Another prior art device for control of vapor deposition systems is an automatic pressure controller. This device is an electromechanical instrument designed to regulate the pressure or gas flow in a vacuum system. The output from a pressure transducer is usually the input to the controller, which in turn controls a servo driven valve or a piezoelectric valve. The servo driven valve itself has a very slow response time. The quartz crystal oscillator and the piezoelectric valve both diminish in stability as response time is reduced, making them only marginally suitable. The quartz crystal oscillator also has a limit on the mass of film which can be added to it, eliminating the use of the quartz crystal in many production applications.
Control devices must have extremely fast response times to be useful in reactive sputtering, since, in most types of reactively sputtered films, the sputtering rate of the elemental or alloy target is much greater than the sputtering rate of the reactively formed compound. During sputtering, a small amount of the reactive gas flowing through the system reacts with portions of the cathode or target surface. If the gas is used up in the reaction, the reactive gas pressure remains low, and the reacted surfaces may be small enough in area that the overall sputtering rate is not affected substantially. However, if gas is introduced to change the elemental ratio of the film compound, enough reactive gas may also be introduced to react at the cathode surface and substantially decrease the sputtering rate. Then, the amount of gas used up in the reaction also decreases. The reactive gas pressure can then increase and can react with more of the surface thus decreasing the sputtering rate more, until the desired elemental ratio is bypassed.
For example, if a metal (M) is being sputtered in the presence of oxygen, the system may be on the metallic edge of the ratio of M to O in the desired intermediate compound. That is, the ratio of M to O may be too high. If the oxygen pressure is increased to obtain the desired intermediate ratio, the increase in oxygen pressure causes more reaction but also causes a decrease in sputtering rate. The decrease in sputtering rate causes less of the oxygen to be used in the reaction and further increases the oxygen pressure, thus forming a cycle beginning with the original increase. This cycle continues until the system is on the oxidized side of the desired ratio, typically in a fraction of a second. Decreasing the oxygen pressure results in a similar reverse cycle until the system is on the metallic side of the desired ratio. The desired intermediate ratio may then be at a point that is impossible to reach by ordinary pressure control apparatus. This cycle of fluctuation represents a basic instability in the region where the desired degree of reaction occurs. The instability becomes more prevalent in a production type of system where a large amount of material is being sputtered.
In producing intermediate films, attempts have been made to circumvent the control problem altogether. For instance, Gillery, in U.S. Pat. No. 3,907,660, describes a process in which the substrate is heated to 204.degree. to 316.degree. C. to inhibit information of the completely oxidized indium-tin film. This process, however, reduces the desirable quality of uniform etchability in the film coating. Other coating methods are described in the background section of the Gillery patent.